Refactor as "Stable Formatting"

[eemeli]

Closes #1
Closes #3
Closes #5
Closes #6

Based on comments from @aphillips and @justingrant, it's probably best to call this a "null locale" and use zxx rather than und as its identifier.

After merging, the repo should also be renamed.

[justingrant]

I think "null locale" is much, much better than "root locale". That said, "null locale" is potentially confused with null (and that's the kind of thing that could lead to long discussion and/or blocking in TG1) so I'd be inclined to continue bikeshedding to see if there's a non-keyword name that could be used instead, like "generic locale".

But changing from root locale to something else is a step in the right direction!

[eemeli]

Ping @zbraniecki, @FrankYFTang, @sffc: This updates the refactor as agreed on the call, and adds a note clarifying that the solution might not lie within Intl.

I'll be renaming the repo as proposal-stable-formatting once this is merged.

[sffc]

TG2 discussion: https://github.com/tc39/ecma402/blob/master/meetings/notes-2023-09-07.md#new-proposal-define-the-behavior-and-properties-of-the-und-root-locale